Step 2

If you can't see the toolbar (it disappears automatically after a few seconds) then tap anywhere in the middle of the screen, and the toolbar will appear.

Tap on the symbol on the left-hand side. A menu will appear.

From the menu, pick "About".

At the bottom of the About Catholic Calendar screen you will see a button marked "Registration Code". Press it.

A web page will open, asking you for your registration code. Follow its instructions.

Afterwards

Once you have gone through this process, your Catholic Calendar will have morphed itself into Universalis. If you like, you can check this by opening the About screen again. It will now say "About Universalis", and at the top it will mention your registration code. Also, there will no longer be a Registration Code button at the bottom of the About screen, because it isn't needed any more.

Note: The icon you tap to get into Universalis will continue to call itself "Catholic Calendar". Don't worry: this is normal. There is no way for an application to change its own icon in Android.